1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134

use rustc_infer::infer::nll_relate::{NormalizationStrategy, TypeRelating, TypeRelatingDelegate};
use rustc_infer::infer::{InferCtxt, NllRegionVariableOrigin};
use rustc_middle::mir::ConstraintCategory;
use rustc_middle::ty::relate::TypeRelation;
use rustc_middle::ty::{self, Const, Ty};
use rustc_trait_selection::traits::query::Fallible;

use crate::borrow_check::constraints::OutlivesConstraint;
use crate::borrow_check::diagnostics::UniverseInfo;
use crate::borrow_check::type_check::{BorrowCheckContext, Locations};

/// Adds sufficient constraints to ensure that `a R b` where `R` depends on `v`:
///
/// - "Covariant" `a <: b`
/// - "Invariant" `a == b`
/// - "Contravariant" `a :> b`
///
/// N.B., the type `a` is permitted to have unresolved inference
/// variables, but not the type `b`.
pub(super) fn relate_types<'tcx>(
    infcx: &InferCtxt<'_, 'tcx>,
    param_env: ty::ParamEnv<'tcx>,
    a: Ty<'tcx>,
    v: ty::Variance,
    b: Ty<'tcx>,
    locations: Locations,
    category: ConstraintCategory,
    borrowck_context: &mut BorrowCheckContext<'_, 'tcx>,
) -> Fallible<()> {
    debug!("relate_types(a={:?}, v={:?}, b={:?}, locations={:?})", a, v, b, locations);
    TypeRelating::new(
        infcx,
        NllTypeRelatingDelegate::new(
            infcx,
            borrowck_context,
            param_env,
            locations,
            category,
            UniverseInfo::relate(a, b),
        ),
        v,
    )
    .relate(a, b)?;
    Ok(())
}

struct NllTypeRelatingDelegate<'me, 'bccx, 'tcx> {
    infcx: &'me InferCtxt<'me, 'tcx>,
    borrowck_context: &'me mut BorrowCheckContext<'bccx, 'tcx>,

    param_env: ty::ParamEnv<'tcx>,

    /// Where (and why) is this relation taking place?
    locations: Locations,

    /// What category do we assign the resulting `'a: 'b` relationships?
    category: ConstraintCategory,

    /// Information so that error reporting knows what types we are relating
    /// when reporting a bound region error.
    universe_info: UniverseInfo<'tcx>,
}

impl NllTypeRelatingDelegate<'me, 'bccx, 'tcx> {
    fn new(
        infcx: &'me InferCtxt<'me, 'tcx>,
        borrowck_context: &'me mut BorrowCheckContext<'bccx, 'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        locations: Locations,
        category: ConstraintCategory,
        universe_info: UniverseInfo<'tcx>,
    ) -> Self {
        Self { infcx, borrowck_context, param_env, locations, category, universe_info }
    }
}

impl TypeRelatingDelegate<'tcx> for NllTypeRelatingDelegate<'_, '_, 'tcx> {
    fn param_env(&self) -> ty::ParamEnv<'tcx> {
        self.param_env
    }

    fn create_next_universe(&mut self) -> ty::UniverseIndex {
        let info_universe =
            self.borrowck_context.constraints.universe_causes.push(self.universe_info.clone());
        let universe = self.infcx.create_next_universe();
        assert_eq!(info_universe, universe);
        universe
    }

    fn next_existential_region_var(&mut self, from_forall: bool) -> ty::Region<'tcx> {
        let origin = NllRegionVariableOrigin::Existential { from_forall };
        self.infcx.next_nll_region_var(origin)
    }

    fn next_placeholder_region(&mut self, placeholder: ty::PlaceholderRegion) -> ty::Region<'tcx> {
        self.borrowck_context.constraints.placeholder_region(self.infcx, placeholder)
    }

    fn generalize_existential(&mut self, universe: ty::UniverseIndex) -> ty::Region<'tcx> {
        self.infcx.next_nll_region_var_in_universe(
            NllRegionVariableOrigin::Existential { from_forall: false },
            universe,
        )
    }

    fn push_outlives(
        &mut self,
        sup: ty::Region<'tcx>,
        sub: ty::Region<'tcx>,
        info: ty::VarianceDiagInfo<'tcx>,
    ) {
        let sub = self.borrowck_context.universal_regions.to_region_vid(sub);
        let sup = self.borrowck_context.universal_regions.to_region_vid(sup);
        self.borrowck_context.constraints.outlives_constraints.push(OutlivesConstraint {
            sup,
            sub,
            locations: self.locations,
            category: self.category,
            variance_info: info,
        });
    }

    // We don't have to worry about the equality of consts during borrow checking
    // as consts always have a static lifetime.
    fn const_equate(&mut self, _a: &'tcx Const<'tcx>, _b: &'tcx Const<'tcx>) {}

    fn normalization() -> NormalizationStrategy {
        NormalizationStrategy::Eager
    }

    fn forbid_inference_vars() -> bool {
        true
    }
}